1. Technical Field
The present invention relates to a projection zoom lens system for a projector for projecting a magnified image displayed on a light valve onto a screen.
2. Description of the Related Art
FIG. 11 shows a schematic structure of a typical three-plate type liquid crystal (LCD) projector. A projector apparatus 8 for projecting a magnified image onto a screen includes a projection zoom lens system 1 for projecting an image supplied from incident side onto the screen, and an image forming apparatus 7 for supplying the image to the projection zoom lens system 1. In the LCD projector, LCD 3 acts as a light valve of the image forming apparatus 7. The projector 8 shown in FIG. 11 includes a white light source 6, dichroic mirrors 5R and 5G and mirror 4 for color-separating the light from the light source 6, liquid crystal panels (LCD) 3R, 3G and 3B. These LCD""s are light through type display medium (light valve) and images of each color, namely red, green and blue are generated respectively. Images formed by these LCD""s 3R, 3G and 3B are introduced into a dichroic prism 2 and color-compounded then led to the incidence side of the projection lens system 1. Accordingly, images displayed on the LCD light valves 3B, 3G and 3R are magnified, compounded and projected onto a screen 9.
The projection zoom lens system in such a LCD projector must have a long back focus for inserting the dichroic prism 2. Since the characteristics of dichroic prism largely depends on an incident angle due to its spectral characteristics, the liquid crystal panel side, the incidence side, of the projection zoom lens system 1 has to be telecentric. Further, not only in the above three-plate type image-forming apparatus, but also in any image-forming apparatus using LCD light valve, it is preferable to apply the projection lens system having the telecentric incidence side. Because the visual angle of the liquid crystal panel is relatively small, the image quality depends highly on the angle.
Recently, in place of the liquid crystal panels, the image forming apparatus including a device having a plurality of elements for forming pixels by mechanically changing reflecting direction of light using micro-electro-mechanical systems have been brought into practical use. One of such apparatus is a DMD (digital mirror device, deformation micro-foil device or display) in which fine mirrors (micro mirrors) are arranged into an array in correspondence with pixels for displaying image by controlling angle of each mirrors. An image forming apparatus including the micro mirrors for forming pixels provides faster response speed and brighter and higher contrast image than those of LCD. Therefore, it is suitable for providing a compact projector of high-intensity and high-quality image.
Further size down of the projector is required for carrying it with a mobile computer. For this purpose, in addition to miniaturize of the panel of light valve, it is essential to miniaturize an optical system such as the projection lens system. However, in the three-plate type projector, since the dichroic prism is disposed between the light valves, such LCD""s or DMD""s, and the projection lens system, the lens system having a long back focus is required. Accordingly, the length of the projection lens system becomes long, which prevents the entire optical system from being made compact.
Moreover, the projection lens system having a large field angle is required for projecting a large image onto the screen. Therefore, large-diameter lens is adopted especially as a lens located closest to the screen. This requirement also conflicts with the miniaturization of the optical system of the projector. Moreover, in order to satisfy the all of requirements that enlarging the lens diameter, improving aberration performance and increasing the relative illumination, it is necessary to constitute the projection lens system with a plenty of lenses. However, use of a large number of lenses introduces many drawbacks such as decreasing of the transmittance of the projection lens as a whole, increasing of the weight and increasing of the cost.
Accordingly, it is an object of the present invention to provide a compact zoom lens system with a simplified lens configuration having high transmittance, high performance and being low-cost while a sufficient back focus required for the projector is secured.
A projection zoom lens system of this invention that has a substantially telecentric incidence side comprises a plurality of lens groups for behaving differently between a wide-angle end and a telephoto end and at least one of two surfaces of a lens included in a first lens group located closest to a screen side has an aspheric surface for simplifying configuration and for realizing a compact and high-performance zoom lens system.
An aspheric lens has been adopted for improving the aberration performance. However, in order to suppress cost increase due to adopting the aspheric lens, the smallest lens located at an intermediate position or a smallest lens located closest to the light valve (incident side) among the lenses constituting the projection lens has been aspheric. On the contrary, in the projection zoom lens system according to the present invention, at least either of surfaces of the lens being located closest to the screen side and having a largest diameter is aspheric.
Since the lens located closest to the screen has a large diameter, the density of light rays for projection becomes minimum among the lenses of the projection zoom lens system. Therefore, by the surface of the lens located closest to the screen, it becomes possible to define the aspheric surface for more properly correcting respective light rays passing through the lens. Therefore, the lens of the first lens group located closest to the screen side having the aspheric surface achieves fine correction and reduces loads of correction of the other lens and/or lens groups of the projection zoom lens system. Accordingly, the number of the lenses constituting other lens group is reduced and the lens configuration of the present projection zoom lens system becomes further simplified.
In addition, since the lens configuration of the projection zoom lens system becomes simple by adopting the present invention, the overall length of the projection zoom lens system becomes shorter. When the length of projection zoom lens system becomes shorter, the diameter of the lens located closest to the screen, which has the largest diameter among the lens system, becomes smaller and minimizes the cost for making the lens having aspheric surface. Therefore, the present invention provides the projection zoom lens system being compact in both length and diameter and having fine correction performance.
Moreover, since the adoption of the aspheric lens in the first lens group corrects the aberration excellently, it becomes possible to arrange the first lens group by the single lens having aspheric surface. As mentioned above, the diameter of the first lens group is large and the cost thereof is high. Therefore, adopting the present invention minimizes the number of larger lenses for the lens system and reduces the overall cost for supplying the lens system. Reducing the number of larger diameter lenses which may requires larger space or length than the smaller diameter lenses, also shortens the overall length of the projection zoom lens system. In addition, the reduction of the number of the lenses increases the transmittance of the projection zoom lens system, and hence it is possible to provide a brighter zoom lens system.
It is preferable that the lens located closest to the screen among the first lens group is a meniscus lens being convex toward the screen side and having a negative refractive power. By arranging the negative meniscus lens at the closest to the screen side, a wide field angle and sufficient relative illumination are secured by the small lens. Therefore, the cost increase of the aspheric surface is further minimized and the diameters of other lens groups following the first lens group become reduced. Consequently, the entire projection zoom lens system that becomes further compact and thin and projects a bright and high-resolution image is provided at a low cost.
The one of the projection zoom lens systems according to the present invention comprises of five lens groups that are negative-positive-positive-negative-positive lens groups, sequentially in this order from the screen side (magnification side) to the incident side (reduction side). That is, the projection zoom lens system comprises the first lens group having a negative refractive power, a second lens group having a positive refractive power, a third lens group having a positive refractive power, a fourth lens group having a negative refractive power and a fifth lens group having a positive refractive power, arranged in this order from the screen side. In this lens system, zooming is carried out by fixing the first and fifth lens groups and moving the second, third and fourth lens groups. The above lens system has the arrangement of the refractive powers for forming basically a retro-focus type, so that an extremely long back focus is secured and it is possible to provide the projection lens system having the incident side being telecentric or very close to that state. Further, since the first lens group has the negative power, the lens system becomes a bright and wide-angle viewing projection zoom lens with the small F-number and the wide field angle. Moreover, by implementing the second and third lens groups so as to move independently, the small-size projection zoom lens system with excellent aberration correction performance is realized.
In the present invention, since the lens located closest to the screen side has the aspheric surface as described above, the aberration correction performance becomes improved. Therefore, the simple lens arrangement of nine to ten lenses in total realizes the projection zoom lens system with excellent aberration correction performance consisting of the five lens groups. Further, the reduction of the number of the lenses improves the transmittance and the total length of the projection zoom lens system as previously explained. Moreover, the projection zoom lens system with the five lens groups constituted less number of lenses makes lightweight and low cost lens system having great imaging or focusing performance.
Therefore, by combining the projection zoom lens according to the present invention with an image forming apparatus for supplying an image for projection to the incident side of this projection zoom lens, a projector being compact, suitable for carrying and capable of projecting a bright and sharp image onto the screen is provided.
In this invention, while the zoom ratio is approximately 1.2 times, it is possible to provide a compact but relatively larger diameter projection zoom lens with an F-number of approximately 2.
In the projection zoom lens system according to the present invention, in order to provide more compact zoom lens system that excels in the correction of aberrations, it is preferable that a composite or synthetic focal length fw12 of the first and second lens groups at the wide-angle end and a synthetic focal length fw of the projection zoom lens system at the wide-angle end satisfy the following condition (A).
3 less than fw12/fwxe2x80x83xe2x80x83(A).
When the focal length fw12 of the first and second lens groups falls below the lower limit, the powers of the first and second lens groups are too strong and hence the distortion, especially distortion at the wide-angle end is not corrected sufficiently.
Further, it is preferable that the overall length L of the projection zoom lens and the focal length f2 of the second lens group satisfy the following condition (B).
1.4 less than L/f2 less than 1.8xe2x80x83xe2x80x83(B).
The focal length f2 of the second lens group that is one of the major variable power groups mainly determines the lens length of the entire projection zoom lens system. When the ratio (L/f2) falls below the lower limit of the condition (B), the overall length of the zoom lens system becomes shorter but it becomes difficult to suppress the coma aberration. On the other hand, when the ratio (L/f2) exceeds the upper limit of the condition (B), the coma aberration becomes extremely small but the lens length of the zoom lens system becomes longer, and hence the miniaturization becomes difficult.
Further, it is important for the focal length f1 of the first lens group and the focal length f2 of the second lens group to satisfy the following condition (C).
0.8 less than |f1/f2| less than 1.3xe2x80x83xe2x80x83(C)
When the absolute ratio |f1/f2| exceeds the upper limit of the condition (C), the back focus Bf becomes too short and the zoom lens system becomes unsuitable as the projection lens (lens for projection). On the other hand, when the ratio |f1/f2| falls below the lower limit of the condition (c), the back focus Bf becomes longer but it becomes difficult to suppress the occurrence of the coma aberration.